This invention relates to a process for producing a high density printed wiring board by an additive process.
Printed wiring boards are generally produced economically by an additive process wherein necessary wiring conductors are formed by electroless plating. According to the additive process, necessary wiring patterns are formed by forming adhesive layers having thereon an electroless plating catalyst such as palladium on surfaces of an insulating substrate having the same catalyst for electroless plating, masking portions other than circuit forming portions with a plating resist selectively roughening an adhesive surface to be formed into circuit portions with a chemical solution such as chromic acid, or the like, neutralizing and washing with water, and forming conductors on circuit portions by electroless copper plating.
With a recent demand for miniaturizing and light weighting electronic devices, printed wiring boards with high density have also been demanded, resulting in narrowing distances between through-holes and between wiring conductors. As a result, by an electric field formed between neighboring wiring conductors or conductors of inner walls of through-holes, various treating solutions retained in surface or inner portions of the insulating substrate supporting the conductors are activated to easily bring about electrolytic corrosion which accelerate migration of conductors. Thus, it is impossible to make the distance between conductors 0.15 mm or less. Further, it is also impossible to narrow the distances between through-holes, so that in order to obtain high density wiring, only the width of conductors should be reduced, resulting in producing a limit to the wiring density. As a cause for bringing about electrolytic corrosion, it is considered that migration of copper is brought about by ionic impurities contained in additive layers formed on the insulating substrate or a residue of a special treating solution used in the additive process when an electric field is applied between wiring conductors under a high temperature and high humidity.
In order to solve the problem of electrolytic corrosion, it is proposed a process comprising conducting electroless nickel plating on whole surfaces of an insulating substrate having adhesive layers thereon and through holes therein, forming a plating resist on portions other than circuit portions, electroplating copper on the circuit portions, peeling the resist, and removing nickel on non-circuit portions with an ammonium persulfate solution containing benzotriazole (Japanese Patent Examined Publication No. 56-47716). This process is illustrated by FIGS. 3(a) to 3(h) wherein numeral 11 denotes an insulating substrate, numeral 12 denotes an adhesive layer, numeral 13 denotes a through-hole, numeral 14 denotes a nickel plated layer, numeral 15 denotes a plating resist, numeral 16 denotes a copper plated layer, and numeral 17 denotes a printed wiring board after removing the nickel layer. But according to this process, since circuit conductors are formed by electroplating of copper on the whole surface of electroless plated nickel, there arise various problems in that the plating thickness in the through-holes obtained by electroplating of copper is not uniform, high density of wiring is difficult due to no room for changing hole diameters at the time of planing or due to no surface smoothness of conductors necessary for mounting various parts thereon, and the like.